Implantable medical devices (IMDs) include devices designed to be implanted into a patient. Some examples of these devices include cardiac function management (CFM) devices such as implantable pacemakers, implantable cardioverter defibrillators (ICDs), cardiac resynchronization therapy devices (CRTs), and devices that include a combination of such capabilities. Such devices can include pacers, defibrillators, cardioverters, cardiac resynchronization therapy (CRT), or various combinations of such devices. Such devices can typically sense intrinsic heart contractions, deliver pacing pulses to evoke responsive heart contractions, or deliver a shock to interrupt certain arrhythmias. This can help improve the patient's heart rhythm or can help coordinate a spatial nature of the heart contraction, either of which may improve cardiac output of blood to help meet the patient's metabolic need for such cardiac output.
For example, detecting a ventricular tachyarrhythmia (e.g., a too-fast ventricular heart rhythm) often involves detecting a rate of ventricular heart contractions that exceeds a tachyarrhythmia rate threshold. By using multiple tachyarrhythmia rate thresholds, multiple tachyarrhythmia rate zones can be established, which can further classify different tachyarrhythmias based on which zone the heart rate falls within.